The present invention relates to a color display tube comprising, in an evacuated envelope, means for generating a number of electron beams, a display screen having a large number of areas luminescing in different colors when struck by the beams, and a color selection structure positioned between the screen and the beam generating means, the structure comprising a multiplicity of apertures for associating each electron beam with luminescent areas of one color, the color selection structure further comprising first and second lens electrode means, situated at a defined distance from each other, for focusing the electron beams passing through each aperture.
Such a display tube is known from U.S. Pat. No. 4,107,569. By applying an electric potential difference between the first and the second lens electrode means, a focusing effect is exerted on the electron beams passing through each of the apertures of the color selection structure. Such display tubes are therefore, sometimes referred to as "post-focusing tubes". The color selection takes place in a manner analogous to that in a color display tube which is equipped with a normal shadow mask. As a result of the electric potential difference between the first and second lens electrode means situated at a short distance from each other, electric flash-over may occur during operation of the tube between an electrode of the first and an electrode of the second lens electrode means. When such a flash-over occurs, the total electric energy stored in the color selection structure may be released in the flash-over point in fractions of a second so that damage may be done to the color selection structure at that area.